Silver-Silver Chloride Electrode And Electrical Circuit

ABSTRACT

A silver-silver chloride electrode contains silicone rubber as a binder in which silver powder, silver chloride powder and silica powder are dispersed. A current density of a current flowing through an electric circuit is equal to or greater than 0.64 μA/mm 2  after 5 minutes from the beginning of voltage application to the electric circuit when the electric circuit in which two silver-silver chloride electrodes and a phosphate buffered saline are connected in series is made up of the two silver-silver chloride electrodes and the phosphate buffered saline containing no calcium and no magnesium interposed between the two silver-silver chloride electrodes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/978,172 filed Sep. 3, 2020, which is a U.S. National Phaseapplication under 35 U.S.C. 371 of International Application No.PCT/JP2019/023195 filed on Jun. 12, 2019, which claims the benefit ofpriority from Japanese Patent Application No. 2018-113792 filed Jun. 14,2018. The entire disclosures of all of the above applications areincorporated herein by reference.

BACKGROUND Technical Field

The present invention relates to silver-silver chloride electrodes andto electric circuits.

Related Art

Silver-silver chloride electrodes are widely used as measurementelectrodes and reference electrodes for measuring minute currents inelectrochemistry and electrophysiology because they are nonpolarizable,have a stable potential, and have a high charge transfer reaction rate.

As a method for producing a silver-silver chloride electrode, a methodis known in which silver chloride is formed by electrolysis on a surfaceof a silver plate or silver wire immersed in a chloride solution.

Furthermore, a method for producing a silver-silver chloride electrodecomposed of silver, silver chloride, and a heat-resistant resin areformed on a substrate is known in which a conductive paste obtained bydispersing silver powder, silver chloride powder, and polyimide (binder)in an organic solvent is applied on the substrate and heated(JP-A-05-142189). Furthermore, JP-A-2005-292022 discloses a method inwhich a paste in which silver particles are dispersed in a resinmaterial is applied to a substrate to form an electrode, and then theelectrode is treated with hypochlorous acid to make a surface of theelectrode silver chloride.

In recent years, studies using microfluidic devices has progressed inelectrochemistry and electrophysiology. For example, it is conceivableto use silver-silver chloride electrodes to measure the microcurrent offluid in a microfluidic device. In this case, a silver-silver chlorideelectrode that has high adhesion to silicone rubber, which is a materialfor a plate used in a microfluidic device, and that can stably maintainhigh conductivity, is desired.

Accordingly, the present invention provides a silver-silver chlorideelectrode having high adhesion to silicone rubber and capable of stablymaintaining high conductivity, and provides an electric circuit havingtwo silver-silver chloride electrodes.

SUMMARY

A silver-silver chloride electrode according to an aspect of the presentinvention is a silver-silver chloride electrode including silver powder,silver chloride powder, silica powder, and silicone rubber as a binderin which silver powder, silver chloride powder and silica powder aredispersed. The current density of the current flowing through anelectric circuit is equal to or greater than 0.64 μA/mm² after 5 minutesfrom the beginning of voltage application to the electric circuit whenthe electric circuit in which two silver-silver chloride electrodes anda phosphate buffered saline are connected in series is made up of thetwo silver-silver chloride electrodes and the phosphate buffered salinecontaining no calcium and no magnesium interposed between the twosilver-silver chloride electrodes.

In this aspect, it is possible to provide a silver-silver chlorideelectrode that has high adhesion to silicone rubber and can stablymaintain high conductivity.

An electric circuit according to an aspect of the present invention isan electric circuit including two silver-silver chloride electrodes anda phosphate buffered saline not containing calcium or magnesiuminterposed between the two silver-silver chloride electrodes, the twosilver-silver chloride electrodes and the phosphate buffered salinebeing connected in series. Each silver-silver chloride electrodeincludes silver powder, silver chloride powder, silica powder, andsilicone rubber as a binder in which silver powder, silver chloridepowder and silica powder are dispersed. The current density of thecurrent flowing through the electric circuit is equal to or greater than0.64 μA/mm² after 5 minutes from the beginning of voltage application tothe electric circuit.

Preferably, the current density of the current flowing through theelectric circuit is equal to or greater than 7.61 μA/mm² after 5 minutesfrom the beginning of voltage application to the electric circuit.

Preferably, the two silver-silver chloride electrodes are formed on asubstrate made of silicone rubber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing silver-silver chloride electrodesmanufactured on a substrate;

FIG. 2 is a table showing materials of multiple samples of silver-silverchloride electrodes;

FIG. 3 is a schematic diagram showing an experimental apparatus fortesting the conductivities of the samples;

FIG. 4 is a graph showing the test results of the conductivities of thesamples;

FIG. 5 is a graph showing the test results of the conductivities of thesamples; and

FIG. 6 is a table showing the test results of the conductivities of thesamples.

DETAILED DESCRIPTION

Hereinafter, an embodiment according to the present invention will bedescribed.

Outline of Embodiment

A silver-silver chloride electrode according to the embodiment containssilver powder, silver chloride powder, silica powder, and siliconerubber as a binder in which silver powder, silver chloride powder andsilica powder are dispersed. In the embodiment, when an electric circuitin which two silver-silver chloride electrodes and a phosphate bufferedsaline are connected in series is made up of the two silver-silverchloride electrodes and the phosphate buffered saline containing nocalcium and no magnesium interposed between the two silver-silverchloride electrodes, the current density of the current flowing throughthe electric circuit is equal to or greater than 0.64 μA/mm² after 5minutes from the beginning of voltage application to the electriccircuit.

A method for manufacturing a silver-silver chloride electrode accordingto the embodiment includes: a step of producing a paste by mixing silverpowder, silver chloride powder, a dispersant, and fumed silica powderwith a liquid silicone rubber binder; a step of coating the paste on asubstrate made of silicone rubber; and a step of curing the paste on thesubstrate to form an electrode in which silver, silver chloride, andsilica powder are dispersed.

Preferably, the manufacturing method includes a step of immersing theformed electrode in a sodium chloride aqueous solution.

The step of producing a paste includes a step of producing a mixture offumed silica powder and silver chloride powder by, first, adding fumedsilica to silver chloride, pulverizing and mixing the silver chlorideand the fumed silica, and a step of adding the mixture, silver powder,and a dispersant to an RTV (Room Temperature Vulcanizing) siliconerubber.

Fumed silica powder functions as an aggregation inhibitor for silverchloride powder. When fumed silica is not used, silver chloride powderagglomerates. Preferably, the fumed silica powder is a hydrophilic fumedsilica powder.

The dispersant disperses silver powder and silver chloride powder asuniformly as possible in a liquid silicone rubber binder. The dispersantis preferably a polyether-modified silicone surfactant having apolyether chain and a silicone chain, and/or a polyglycerin-modifiedsilicone surfactant having a polyglycerin chain and a silicone chain.

In the step of coating the substrate with the paste, as shown in FIG. 1, a surface of the substrate 1 made of silicone rubber is coated withthe paste 2 by a technique such as screen printing or ink jet printing.Curing of the paste 2 results in silver-silver chloride electrodes 3 inwhich silver, silver chloride, and silica powder are dispersed. In otherwords, a plate 4 having the silver-silver chloride electrodes 3 providedon a surface is manufactured.

Preferably, the silver-silver chloride electrodes 3 are produced byimmersing the silver-silver chloride electrodes in a sodium chlorideaqueous solution and drying them.

In the illustrated embodiment, two silver-silver chloride electrodes 3are formed on one surface of the substrate 1. However, one or three ormore silver-silver chloride electrodes 3 may be formed on the substrate1, or one or more silver-silver chloride electrodes 3 may be formed onboth surfaces of the substrate 1.

In accordance with the silver-silver chloride electrode produced by thisproduction method, in a case in which hydrophilic fumed silica powder isused, it is assumed that the affinity between the surfaces of silverchloride particles and electrolytes (for example, electrolytes in asolution to be measured) is improved since the surface of each silverchloride particle is coated with hydrophilic fumed silica. Although theconductivity of silver chloride itself is low, it is considered that theconductivity is improved by coating silver chloride particles withhydrophilic fumed silica. In addition, it is assumed that since thedispersant disperses silver powder and silver chloride powder, which areconductors, in silicone rubber, which is the binder, the conductorparticles within the silver-silver chloride electrode are electricallyconnected to one other well, so that conductivity is also improved.

Furthermore, the silicone rubber contained in the silver-silver chlorideelectrode contains chloride ions and sodium ions derived from sodiumchloride if the step of immersing in a sodium chloride aqueous solutionis conducted. Therefore, it is assumed that the conductivity is improvedby ions in addition to the electrical connection of the conductorparticles, so that a higher conductivity can be stably maintained.

In addition, by using silicone rubber as a binder, the producedsilver-silver chloride electrode 3 has high adhesion to the siliconerubber and does not easily peel off or drop off from the substrate 1.Furthermore, since the silicone rubber contained in the silver-silverchloride electrode contains chloride ions and sodium ions derived fromsodium chloride, it is expected to improve durability against externalforces caused by, e.g., bending of the silver-silver chloride electrode.

Production Examples

The inventor manufactured multiple samples each having silver-silverchloride electrodes by the manufacturing method according to theembodiment, and tested the conductivities of these samples. Forcomparison, a sample (Sample 10) having silver electrodes was produced,and the conductivity of the sample was also tested.

FIG. 2 shows the materials of these samples and details of immersion ina sodium chloride aqueous solution (salt water treatment). In FIG. 2 ,unless otherwise noted, the numerical values represent parts by weight.The “%” in the last line (salt water treatment) indicates theconcentration of sodium chloride in the sodium chloride aqueous solutionas a percentage, whereas “None” in the last line indicates that theelectrodes were intentionally manufactured without performing the saltwater treatment. The “−” in the last line indicates that the salt watertreatment was abandoned, and that the conductivity test was notperformed.

For Samples 1-9, in the step of producing a mixture of fumed silicapowder and silver chloride powder, fumed silica was added to silverchloride, and then, silver chloride and fumed silica were pulverized andmixed by means of a centrifugal mill. For Samples 1 to 9, the weightparts of silver chloride and fumed silica in the entire material are asshown in FIG. 2 . The raw material silver chloride was produced byInuisho Precious Metals Co., Ltd., Osaka, Japan. As the fumed silica,there were prepared “AEROSIL 200”, which is a hydrophilic fumed silicamanufactured by Nippon Aerosil Co., Ltd., Tokyo, Japan, and “AEROSILR972” which is a hydrophobic fumed silica manufactured by the samecompany. “AEROSIL R972” was used for the manufacture of Samples 2 and 7,whereas “AEROSIL 200” was used for the manufacture of Samples 1, 3-6, 8,and 9. “AEROSIL” is a registered trademark. For pulverization andmixing, a centrifugal mill (trade name “ZM 200”) manufactured by RetschCo., Ltd. (currently Verder Scientific Co., Ltd.), Tokyo, Japan wasused. Silver chloride and fumed silica were pulverized and mixed, sothat the resulting particles passed through a 0.20 mm-mesh screen.

In samples 10 to 12, no fumed silica powder was used. The reason for notusing fumed silica powder in Samples 11 and 12 was to confirm the effectof fumed silica powder as an aggregation inhibitor for silver chloridepowder. The reason for not using fumed silica powder in Sample 10 wasthat no silver chloride powder was used, and therefore, fumed silicapowder as an aggregation inhibitor was unnecessary.

For silicone rubber as the binder, a mixture of “KE-106”, an RTVsilicone rubber manufactured by Shin-Etsu Chemical Co., Ltd., Tokyo,Japan and “CAT-RG”, a curing catalyst manufactured by the same company,was used.

As silver powder, there were prepared a flaky silver powder, “FA-2-3”,manufactured by Dowa Hitech Co., Ltd., Saitama, Japan, and anirregular-shaped silver powder, “G-35” manufactured by the same company.Equal amounts of these were used in each of the samples.

As the dispersant, there were prepared polyether-modified siliconesurfactant, “KF-6015” manufactured by Shin-Etsu Chemical Co., Ltd., andpolyglycerin-modified silicone surfactant, “KF-6106”, manufactured bythe same company. Equal amounts of these were used in each of thesamples.

For Samples 1 to 9, a paste was produced by adding silver powder, thedispersant, and the mixture of fumed silica powder and silver chloridepowder to the binder and mixing them.

For Samples 11 and 12, silver powder, the dispersant, and silverchloride powder were added to the binder, and they were mixed, but sincethey did not contain fumed silica powder as an aggregation inhibitor forsilver chloride powder, silver chloride powder agglomerated and auniform paste could not be produced (thus, Samples 11 and 12 were notsubjected to subsequent steps and to the test. In FIG. 2 , the “−” insalt water treatment for Samples 11 and 12 means that neither the saltwater treatment nor the conductivity test was conducted due to the pastebeing inferior). Samples 11 and 12 differed in the amount of silverchloride, but none of them could result in production of a uniformpaste. Thus, the effect of fumed silica was confirmed.

For Sample 10, a paste was produced by adding silver powder and thedispersant to the binder and mixing them.

Then, for Samples 1 to 10, as shown in FIG. 1 , the paste 2 was coatedby screen printing at two locations on a surface of a substrate 1 madeof silicone rubber containing PDMS (polydimethylsiloxane). Furthermore,the paste 2 was cured by heating at 150 degrees Celsius for 30 minutes.

For Samples 3 to 7, and 10, except for Samples 1, 2, 8, and 9, aftercuring the paste 2, the substrate 1 was immersed in a sodium chlorideaqueous solution at room temperature for an hour together with theelectrodes resulting from the paste 2, and they were then dried.

In each of produced Samples 1 to 9, the silver-silver chlorideelectrodes 3 had high adhesion to the silicone rubber and did not easilypeel off or drop off from the substrate 1. Furthermore, in Sample 10manufactured for comparison, the silver electrodes 3 had high adhesionto the silicone rubber and did not easily peel off or drop off from thesubstrate 1. In these samples, the length L of the electrodes 3 was 30mm, the width W thereof was 5 mm, and the interval IN therebetween was10 mm.

Next, using each of produced Samples 1 to 10, an experimental apparatus5 shown in FIG. 3 was assembled. The experimental apparatus 5 has plates4, 6, and 7 that are stacked and bonded to one another. Through-holes 6a and 6 b are formed in the plate 6 immediately above the plate 4, andare overlapped with the electrodes 3, respectively. In the uppermostplate 7, a groove 7 g that penetrates the plate 7 is formed. One end ofthe groove 7 g is overlapped with the through-hole 6 a of the plate 6directly below, whereas the other end of the groove 7 g is overlappedwith the through-hole 6 b.

Thus, the experimental apparatus 5 is provided with a micro flow channelhaving the through-holes 6 a and 6 b and the groove 7 g. Both ends ofthe micro flow channel are closed with the two electrodes 3. Liquid canbe stored in the micro flow channel, and liquid can be introducedthrough the groove 7 g. The width of the groove 7 g was 1 mm, whereasthe diameters of the through-holes 6 a and 6 b were 2 mm.

PBS (phosphate buffered saline) was supplied to the micro flow channelfrom the groove 7 g. The PBS used was PBS (−) without calcium ormagnesium.

A battery 8 (DC power supply) was connected to the electrodes 3 on thesurface of the plate 4 via lead wires L, and a voltage of 0.3V wasapplied so that a DC current flowed through the electrodes 3. Variationof the electric current value was measured by an ammeter 9 for 400seconds (6 minutes and 40 seconds) immediately after the beginning ofelectric current supply (voltage application).

Therefore, an electric circuit having two electrodes 3 and PBS (−)therebetween was formed in which the electrodes 3 and PBS (−) wereconnected in series.

FIGS. 4 and 5 show the measurement results. The measurement results inFIGS. 4 and 5 are the first measurement results after the plates 4 weremanufactured.

As is clear from FIGS. 4 and 5 , in Samples 4 and 5, the current valuewas stabilized for 400 seconds after the beginning of voltageapplication. Samples 4 and 5 contain hydrophilic fumed silica, and thesodium chloride concentration of the solution used in the salt watertreatment is high. It is presumed that the silicone rubber contained inthe silver-silver chloride electrodes in Samples 4 and 5 contain a largeamount of chloride ions and sodium ions derived from sodium chloride, sothat the conductivity is improved and the higher conductivity can bestably maintained by the ions.

As is clear from comparison of Samples 4 and 5, even though the silverchloride content was different, the current value was stable for a longtime if the sodium chloride concentration of the solution used in thesalt water treatment was higher.

In Samples 1 to 3 and 6 to 9, a large current flowed immediately afterthe beginning of voltage application, but the current value decreasedwith time.

Sample 1 used the same materials as Sample 4, but was not subjected tothe salt water treatment. In sample 1, the current value graduallydecreased with time.

Samples 3 and 6 used the same materials as Sample 5, but the sodiumchloride concentration of the solution used in the salt water treatmentwas low for Sample 3, and Sample 6 was not subjected to the salt watertreatment. In Samples 3 and 6, the current value gradually decreased andthen stabilized.

Sample 7 used the same materials as sample 5, but used hydrophobic fumedsilica instead of hydrophilic fumed silica. In Sample 7, a very largecurrent flowed immediately after the beginning of voltage application,but the current value gradually decreased and then stabilized.

Sample 2 used the same materials as Sample 7, but was not subjected tothe salt water treatment. In sample 2, the current value decreasedrapidly in the initial stage and then stabilized. In Sample 2, thecurrent flowing was smaller than that of Sample 7.

Samples 8 and 9 used the same materials as Sample 5, but the ratio ofhydrophilic fumed silica was low and the salt water treatment was notperformed. In Samples 8 and 9, the current value gradually decreased andthen stabilized.

In Sample 10 having the silver electrodes 3 manufactured for comparison,the current values were lower continuously after the beginning ofvoltage application than those of Samples 1 to 9 having thesilver-silver chloride electrodes 3.

From the above, it is understood that among the samples having thesilver-silver chloride electrodes 3, Samples 4 and 5 had goodperformance.

In microfluidic devices, from the viewpoint of shortening themeasurement time, it is required that electrodes have high conductivityimmediately after the beginning of voltage application. Even Samples 1to 3 and 6 to 9 having a large decrease in current can also be usedutilizing the high conductivity, as long as the measurement is for ashort time. Accordingly, FIG. 6 shows the current value for each sampleat 300 seconds (5 minutes) after the beginning of voltage applicationobtained from the measurement results. Moreover, FIG. 6 shows thecurrent density for each sample at 300 seconds (5 minutes) after thebeginning of voltage application from a measurement result foruniversalization. The current density was obtained by dividing thecurrent value by the cross-sectional area of the lead wires L. Since thelead wires L had a diameter of 2 mm, the cross-sectional area thereofwas 3.14 mm².

Since Samples 1 to 3 and 6 to 9 can be used in microfluidic devices, itis preferable that the current density of the current flowing throughthe electric circuit be equal to or greater than 0.64 μA/mm² after 5minutes from the beginning of voltage application to the electriccircuit.

In consideration of the good performance of Samples 4 and 5, it is morepreferable that the current density of the current flowing through theelectric circuit be equal to or greater than 7.61 μA/mm² after 5 minutesfrom the beginning of voltage application to the electric circuit.

Although the present invention has been described above, the foregoingdescription is not intended to limit the present invention. Variousmodifications including omission, addition, and substitution ofstructural elements may be made within the scope of the presentinvention.

What is claimed is:
 1. A method for producing a silver-silver chlorideelectrode, comprising: simultaneously pulverizing silver chloride powderand silica powder; mixing the pulverized silver chloride powder andsilica powder to produce a mixture of the silver chloride powder andsilica powder; mixing the mixture of the silver chloride powder andsilica powder, silver powder, and a dispersant with a liquid siliconerubber binder to produce a paste; coating the paste on a substrate madeof silicone rubber; and curing the paste on the substrate to form anelectrode containing silver and silver chloride.
 2. The method accordingto claim 1, further comprising passing the mixture of the silverchloride powder and silica powder through a 0.20 mm-mesh screen beforeproducing the paste.
 3. The method according to claim 1, wherein thesilica powder is a hydrophilic fumed silica powder.